Conducting polymers have generated great interest because of their moderate mobilities and ability to change optical properties reversibly. Commercial products from conducting polymers are potentially cost effective, more easily processed, lighter in weight, and more flexible, than products fabricated from alternate material in existing technologies. A class of conducting polymers, polyheterocyclics, which include polythiophenes, polypyrroles, and polyfurans, are a well known class of conducting polymers. More specifically, the conducting polymers, poly(3,4-alkylenedioxyheterocyclics) have been extensively studied in electrochromic devices, photovoltaic devices, transparent conductors, antistatic coatings, and as the hole transport layer in light emitting diodes. The 3,4-alkylenedioxy bridge on the heterocycle allows a modified polyheterocycle where the bridge does not cause an undesirable conformational change in the backbone of the polymer and the electron donating effect of the oxygen substituents increases the HOMO of the conjugated polymer reducing its band gap.
A drawback to the processing of unsubstituted poly(3,4-alkylenedioxyheterocyclics) for example poly(3,4-ethylenedioxythiophene), or poly(3,4-alkylenedioxyheterocyclics) that have small or highly polar substituents, results from their poor solubility. However, the poor solubility is a desirable feature after processing of the conducting polymer in many of the applications of the manufactured devices containing these conducting polymers. Processing methods that render a soluble film insoluble are useful in multilayer device architectures, such as organic light emitting diodes and photovoltaic cells. The processing of poly(3,4-alkylenedioxythiophenes) has relied mostly on depositing an aqueous dispersion of the oxidized form of alkylenedioxythiophenes in the presence of polymer electrolytes, such as poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate. However, the acidic nature of the polystyrene sulfonate polyelectrolyte has adverse effects on device quality.
The solubility of poly(3,4-alkylenedioxythiophenes) can be enhanced by the presence of substituents that interact with a desired solvent. For example, substitution of poly(3,4-ethylenedioxythiophene) on the ethylene bridge with long alkyl side chains solubilizes the polymer without a significant increase of the band gap relative to the unsubstituted polymer. A recent approach for processing polythiophenes is to deposit a soluble polythiophene, due to the presence of an appropriate substituent, on a device substrate followed by the rendering of the polythiophene into a insoluble state by removal of the substituent. Although this approach is inherently desirable, the manners in which it has been achieved have some drawbacks. Efficient cost effective methods of converting a soluble poly(3,4-alkylenedioxyheterocycle) to an insoluble poly(3,4-alkylenedioxyheterocycle) remain desirable to facilitate the fabrication of devices that use these conducting polymers.
Holdcroft et. al. (Chemistry of Materials 2002, 14, 3705) discloses substituent cleavage in copolymers of dioxythiophenes to change a soluble alternating copolymer film into an insoluble film. Tetrahydropyranyl groups are used to protect alcohol functionalities on a 11-hydroxyundecyl substituted thiophene repeating unit which alternates with a ethylenedioxythiophene repeating unit. The tetrahydropyranyl groups cleave by the use of an acid catalyst upon heating 130° C. or greater. Subsequent washing of the polymer film with chloroform removes the untreated polymer leaving the deprotected alcohol substituted polymer in place. A disadvantages of this method for many applications is the use of a copolymer where the electron rich dioxythiophene repeating unit is diluted with a thiophene repeating unit as this reduces the quality of the electrooptical properties from that of a poly(alkylenedioxythiophene) homopolymer. Another disadvantage of this method is the requirement of high temperatures that can lead to copolymer damage, decreasing the lifetime of electronic devices fabricated from this copolymer. Furthermore, the addition of an acid can effect the lifetime and quality of the electronic device.
Shashidhar et. al. (Synthetic Metals 2004, 144, 101.) discloses side group cleavage of a single perflourinated ester substituted poly(3,4-ethylenedioxythiophene) to increase conductivity. The polymer is synthesized by making a solution of monomer in oxidizing agent and imidazole and spin casting the solution on a substrate followed by heating the film to 110° C. Polymerization in the absence of imidazole is also disclosed where an insoluble and insulating film result upon polymerization. Subsequent immersion of the film in imidazole solution cleaves the ester side groups. Disadvantages of this method include: the use of a perfluorinated acid equivalent, which considerably raises the cost of the polymer and processing to include environmental controls; the resulting polymer can not be regular or symmetric, which can reduce the durability of a polymer relative to a regular or symmetric polymer; and the polymerization is performed on the substrate rendering the oxidized polymer directly and restricts the use of this approach from many applications where the neutral polymer rather than the conducting polymer is desired.
Hence the need remains for a method of converting a soluble poly(3,4-alkylenedioxyheterocycle) to an insoluble poly(3,4-alkylenedioxyheterocycle) where an insoluble, unoxidized regular homopolymer is needed at a reasonable cost and temperature. A desirable method would also fully remove dopants, oxidizing agents, and short oligomeric chains which can decrease the quality and shorten the lifetime of an electronic device fabricated with the poly(3,4-alkylenedioxyheterocycle).